Remove http-client crate, suggest reqwest as a default client

[simolus3]

Like all PowerSync SDKs, we need a way to send HTTP requests with streaming responses here. Previously, we've used the http-client crate as an interface for HTTP clients and relied on a libcurl wrapper as a default implementation.

While that generally worked pretty well, the http-client crate is not that actively maintained and most of the Rust ecosystem (also Tauri in particular) seems to use reqwest as a default client. We could implement the http-client trait via reqwest, but there are some downsides for that. In particular, reqwest represents response bodies as a Stream<Bytes> while http-client represents them as an AsyncBufRead. That's a gap that can be bridged, but since we kind of want to use whatever reqwest does by default, it's easier to just remove the http-client dependency and suggest reqwest instead.

To be able to use the SDK with custom clients, reqwest is an optional dependency and it's possible to implement custom http clients (I've introduced a trait exposing just the functionality needed by the SDK, that's also what we use for mocks in tests).